1. Field of the Invention
The present invention relates to an information retrieval apparatus incorporated in a microfilm reader, a reader/printer, or the like and, more particularly, to an information retrieval apparatus for detecting a mark recorded on an information recording medium such as microfilm and retrieving desired information.
2. Related Background Art
In a conventional retrieval apparatus incorporated in a microfilm reader or the like, a microfilm having marks respectively at the sides of image frames is used, and each mark is photoelectrically detected. The detected marks are counted to automatically retrieve a desired frame of the microfilm.
As shown in FIGS. 23 and 24, a mark m on a microfilm F is illuminated with a lamp 100, and a change in light which is shielded by the mark m is detected by a mark detector 101. An output signal from the mark detector 101 is counted by a counter in a controller 102. A count of the counter is compared by a comparator with a frame number of a desired image which is input from a console 103 such as a keyboard. If the count coincides with the input frame number, a stop signal is output from the comparator to a motor drive control circuit 104 to stop a motor M1 serving as a drive system for feeding the microfilm F. The desired frame is stopped at a predetermined position on a screen 105 through a projection lens 106.
As shown in FIGS. 10A, 10B, 10C and 10D, marks m are often recorded at different positions by different projectors. In a conventional information retrieval apparatus, the mark detector 101 is moved in accordance with positional differences in marks m so that a microfilm stop position is set to be an optimal position. FIG. 25 shows a structure of the mark detector 101. Light from the lamp 100 is received from a distal end 110a of an optical fiber 110 through a condenser lens 107 and a microfilm F and is guided to a photosensor 111. The mark detector 101 as a unit can be moved and adjusted in a direction of an arrow x along a guide groove 112 upon rotation of a screw shaft 113. The mark detector 101 can be adjusted to be located immediately above the mark m on the microfilm F.
At this time, since the mark detector 101 is located near an optical axis l of an optical system, as shown in FIGS. 23 and 24, the detector 101 is projected as a shadow S' on the screen 105. The position of the mark m recorded on the microfilm F is projected on the screen 105, and an operator turns an adjusting knob 114 such that the mark detector 101 is located at an optimal position while observing the screen 105. Therefore, the detector 101 can be set at the optimal position.
In the conventional apparatus, however, since the mark detector 101 is located near the optical axis l of an illumination optical system, its shadow S' is projected on the screen 105 and may conceal the image during a read operation of a frame 120 or interferes with an operator's field of view.
In the conventional apparatus, the lamp 100 for illuminating an image recorded in the frame 120 onto the screen 105 is also used as a light source for detecting the mark m. When the operator optimally adjusts the brightness of the screen illumination at the time of projection of the image on the screen 105, the amount of light incident on the mark detection photosensor 111 is also changed. An optimal light amount level for the photosensor 111 is not necessarily obtained. It is therefore difficult to stably operate the apparatus.
Furthermore, shading and a nonuniform light distribution are present in an actual illumination optical system. When the illumination optical system is used as a light source for detecting a mark as in the conventional case. The output from the mark detector 101 becomes unstable due to shading and the nonuniform light distribution. Shading is defined such that the quantity of light is generally increased when an object comes closer to the optical axis. Shading occurs due to various causes such as the state of a lamp filament, optical axis misalignment, dust, and contamination.
In the conventional technique described above, a light bulb such as a halogen lamp is used as the lamp 100 for illuminating the image and the mark, and therefore the following problems are presented.
(1) Since light from the lamp 100 has an emission wavelength range from a near infrared range of 700 nm or more to an infrared range, light is transmitted even through a mark (i.e., a nontransparent portion) of the microfilm, and a contrast level becomes low. As a result, the mark cannot be accurately or properly detected.
(2) In order to solve the above problem, an optical filter must be used in a conventional arrangement, and the entire mechanism becomes bulky and complicated, resulting in a high cost. In addition, it is difficult to perfectly eliminate the above-mentioned wavelength components.
(3) Since substantial heat is generated by a light bulb such as a halogen lamp, the temperature of the photosensor for detecting this light is increased accordingly. The characteristics of the photosensor as a semiconductor sensor are adversely affected to cause an operational failure of the sensor.
(4) Since the light bulb is used, the service life of the apparatus is short, and a serviceman must frequently replace the light bulb with a new one. For this purpose, the apparatus must be designed so that the light bulb can be easily replaced.
(5) The characteristics of the light bulb are subjected to deterioration over time, and the amount of light emitted is undesirably changed during use.
(6) When the quantity of light emitted is automatically adjusted, a CPU (microcomputer) or the like must be used. When a variable current is used, a time lag (long response time) between the change in current and a change in the temperature of the filament of the light bulb occurs to undesirably prolong the automatic light amount setting operation. For example, when the quantity of light is detected by the photosensor and is controlled to be a predetermined value, it takes about 5 to 6 seconds (a long response time in light amount adjustment).
(7) When a light bulb such as a halogen lamp is used, the drive power is increased and a large light source power circuit is required. The space for mounting this circuit is also increased, and the apparatus as a whole becomes bulky. In addition, a large-capacity drive transistor must be used, and its heat dissipation must be taken into consideration. As a result, the apparatus becomes complicated and the frequency of operational failure is increased, thus resulting in a high cost.
(8) When a blue diazo microfilm is used, light is transmitted at about a wavelength of 350 to 550 nm, and the contrast level is undesirably lowered.